The long term goal of this research is to use H3N8 influenza virus as a model to determine viral genetic mutations in single or multiple gene segments of influenza virus A that facilitate interspecies transmission and induction of severe pulmonary disease. The hypothesis being pursued in these studies is: Transmission of equine influenza virus to dogs and adaptation in the new host to induce severe respiratory disease was characterized by mutations in one or several gene segments that allowed the virus to replicate to a greater extent in macrophages and induce higher levels of TNF-. The specific aims of this pilot research project are: 1)To compare virus replication and induction of TNF- mRNA and protein synthesis and other cytokines in canine alveolar macrophages following inoculation with H3N8 canine influenza viruses from 2003-2004 with those following inoculation with H3N8 equine influenza virus isolates ranging from 2003 to 1991; 2) To determine whether the canine influenza virus derived with a reverse genetics system (canine/FL/04-rg) replicates to similar extent and induces comparable levels of TNF- mRNA and protein in canine alveolar macrophages as wild type virus (A/canine/Florida/43/2004) does; and 3) To compare the capacity of canine and equine influenza virus isolates to replicate in primary airway epithelial cells and induce chemokines (IL-8 and MCP-2). The project will use primary isolated dog alveolar macrophages inoculated in vitro with equine influenza A viruses isolated from 1991 through 2003 and initial canine influenza isolates from 2003 and 2004. TNF- mRNA and protein will be measured with quantitative real-time PCR and antigen capture assays, respectively. Virus production will be measured by infectious plaque assay and virus matrix gene quantitative real-time PCR.